Optimize Those Last Few Irrigations

Pivot in field — Irrigating a late season soybean field. (Photo by Steve Melvin)

August 17, 2023

Steve Melvin - Extension Educator Irrigated Cropping Systems

Crystal Powers - Water and Cropping Systems Extension Educator, Nebraska Water Center

Key Takeaways

Leave 40% of plant available water at the end of the season to store off-season precipitation.
Benefits: save money, decrease nitrogen leaching, drier soil for harvest, saving water allocation.

Scheduling the last few irrigations of the season deserves extra attention because the goal is not only to focus on keeping the crop wet enough to produce optimal yields, but also on using up stored soil water. Leaving the field a little drier at the end of the season will save irrigation costs, decrease leaching losses, improve soil conditions for harvest traffic, and save water for future years. Growers also don’t want to miss out on capturing off-season precipitation. Lowering the soil moisture level to 40% of plant available water in the top four feet will give about 2.4 inches of off-season water storage in sandy soils and about 5.5 inches in silt loam soils without lowering yields.

University of Nebraska Extension irrigation scheduling recommendations encourage irrigators to allow the crop to continue using more and more of the stored soil water starting in August and continuing into September when the crop matures (Figure 1). The recommendation is to lower the soil water level from the usual summer water condition of no less than 50% plant available water in the top three feet of soil to 40% in the top four feet after the R4 stage is reached — dough for corn, and end of pod elongation for soybean. Thus, the stored soil water content should be significantly lower when the crop matures in September than earlier in August.

**Figure 1a.** Long-term daily average and individual year corn water use in Nebraska by growth stages and date. (UNL G1850)

**Figure 1b.** Long-term daily average and individual year soybean water use in Nebraska by growth stages and date. (UNL G1367)

According to recent data, many fields are missing this opportunity to improve water use. These fields end the season with fairly wet soil with little to no storage room for off-season precipitation. Each year, the Upper Big Blue NRD requires each farmer in six high nitrate zones across the district to use soil water monitoring equipment in at least one irrigated field. The study analyzed data from the fields that used a Watermark system, which includes three sensors placed at different depths to represent the root zone of corn and soybeans, and a data logger to automatically record the data.

Using the 40% recommendation, the data shows many fields are applying more water late in the season than is needed. Some years, a significant rain can cause the soil to be wetter in September, but it is usually due to applying more irrigation water after the middle of August than needed. The data shows that in 2017, 72% of fields were over-irrigated late in the season. Even in the drought year of 2022, 36% of fields were over-irrigated late in the year.

Table 1. The percentage of fields that became wetter (over-irrigated) moving from August to Sept. 15. The dry years (‘20, ’21, ’22) are much drier than the other years.
			2017	2018	2019	2020	2021	2022	Weighted Avg, Normal to Wet Years ‘17,’18,’19	Weighted Avg, Dry Years ‘20,’21,’22
Soil in the 15-25“ zone			72%	58%	54%	45%	44%	36%	65%	42%
Soil in the 25-36” zone			75%	58%	54%	21%	48%	20%	66%	30%

The data did not ask about late-season decision-making, but it could be because we are all creatures of habit. The irrigation routine is set in July when the plants are transpiring at their peak, the days are long, and the temperatures are high. Then, as the daylight hours shorten (by about three hours by Aug. 20) and the temperatures get cooler in late summer, many keep irrigating at July levels even though crop water use for corn has gone from an average of two inches/week at silking to 1.25 inches/week at full dent (Figure 1). Other crops, including soybeans, have a similar dramatic drop in crop water use moving through August and into September.

The advantages to reducing soil water include saving money on pumping costs, leaving room to store the off-season precipitation, improving field conditions for harvest, saving water allocation, and reducing the potential for leaching nutrients like nitrate deeper into the profile. Even if your irrigation costs are only $10/ac-in, each inch reduction is worth about $1,300 for each quarter-section pivot. For example: A field of silt loam soil, if it is left at field capacity, is missing out on 5.5 inches of off-season storage, worth about $7,200 — not even accounting for the leaching loss of nutrients.

For most of Nebraska, adequate precipitation will be received from October through May to refill the soil profile on irrigated fields. The Grand Island area gets about 14.2 inches during this timeframe and even in Scottsbluff, they still receive about 8.6 inches on average. In addition, leaving the soil drier will help reduce harvest delays and compaction because of mud in wetter falls. For fields with water allocations or shallow aquifers, these water savings can be critical during multi-year droughts.

Reducing nitrate losses to groundwater is another key advantage of optimizing field management. Effective leaching reduction combines precision nutrient and water management. Residual nitrogen is at risk for loss when off-season moisture moves through the soil profile. So, the goal for each field is to match both the nitrogen and water needs for the crop.

So just how much water do we need to finish out the growing season? Table 2 gives you a good idea of the water needed based on crop growth stage. For more information, see NebGuide G1871.

Table 2. Crop water use for the remainder of the growing season for corn and soybean.
Stage of Growth	Approximate Days to Maturity	Water Use to Maturity
Corn
R4 - Dough	34	7.5
R4.78 - Beginning dent	24	5.0
R5 - 1/4 milk line	19	3.75
R5 - 1/2 milk line	13	2.25
R5 - 3/4 milk line	7	1.0
R6 - Physiological maturity	0	0
Soybean
R4 - End of pod elongation	37	9.0
R5 - Beginning seed enlargement	29	6.5
R6 - End of seed enlargement	18	3.5
R6.5 - Leaves begin to yellow	10	1.9
R7 - Beginning maturity	0	0

During the heart of the irrigation season, we recommend keeping the available soil water level above 50% of plant available water. To do this, we recommend irrigating as the soil water level approaches 65% of plant available water. This will allow a few days for the irrigation to be completed before the crop experiences any stress. As we near the end of the season, we can push the threshold to 40% of plant available water.

Corn at the beginning dent stage needs five inches of water to reach maturity (Table 2). Using the silt loam soil from above, the example field would have enough water to reach maturity and have an estimated 0.3 inches to spare if the corn is beginning to dent now (5.3 – 5.0 = 0.3 inches). The loamy sand, at field capacity, would have 2.6 inches available above the 40% (60% depletion) level. This field would need an additional 2.4 inches of water to reach maturity.

One thing to note is that the time needed for corn to mature is dependent on growing degree days. If corn needs five inches of water to reach maturity and we receive some hot windy days in late August, the corn will still only use five inches — it will just finish up a few days quicker.

In contrast, soybean maturity is dependent on day length. Because soybeans may use more or less water than the averages listed in the table, and because it may be difficult to determine the actual correct growth stage, it is important to continue to monitor soil water until maturity.

This is where tools such as an ET gage and soil water sensors come into play. An ET gage will give you potential crop water use and the soil water sensors will give you an idea of how much water is stored in the soil profile. Then you will be able to determine how much water the crop will need in either irrigation or precipitation to finish out the year.